'Through the storm'

He insisted he would remain an "active" member of the BNP and be on hand to offer advice to Mr Walker and whoever was elected leader in a leadership ballot due to take place next year.

Mr Griffin has often been a divisive figure during his 15 years at the top of the party

"The decision was mine and mine alone, though it involved a lengthy and constructive discussion between the entire collective leadership," he wrote.

He suggested the party was in a more stable financial and political position than it had been for some time.

"I had initially hoped to hand the responsibility over several years ago, but when a concerted effort was made to destroy the BNP from both outside and within, I decided that it was my duty to stay and steer our movement through the storm," he said.

"The leadership team is united as to the way ahead and, once again, the BNP - for all that there are many improvements to be made - is the only effectively functioning, genuine nationalist game in town."

Electoral success

Mr Griffin has been the public face of the BNP for more than a decade, during which time the party increased its popular support despite being dogged by allegations of racism, which it has always rejected.

Under his leadership, the party's vote in European elections had increased from 1.1% in 1999 to 3.9% in 2004 and 6.2% in 2009, when Mr Griffin was himself elected as an MEP.

In 2009, Mr Griffin was invited to appear on Question Time, the BBC's flagship political discussion show, a decision which led to widespread demonstrations at the time.

In recent years, the party's fortunes have declined steeply amid internal fighting.

Andrew Brons, the BNP's other representative in the European Parliament, quit the party in 2012 to become an independent, while Mr Griffin was challenged for his position last year.

Mr Griffin, who was declared bankrupt earlier this year, failed to secure re-election as MEP for the North West of England in May as the party lost its deposit in all the UK's electoral regions in which it fielded candidates.

'United'

The BNP blamed its poor performance on the rise of UKIP, which it said had become the party of choice for protest votes about Europe and immigration.

In his statement, Mr Griffin said while it would be wrong to blame him solely for the party's electoral slump, he accepted responsibility and stepping down would prevent the risk of "months of internal friction".

And he defended his track record during his years in charge.

"A party which, until I took it over, had only ever held a single council seat for a few months, is now on course to have had unbroken elected presence in British politics for 17 years," he said.

In its own statement, the BNP said Mr Walker, from Spennymoor, County Durham, "has accepted the role of acting chairman of the British National Party after Nick Griffin stepped aside"

"The full national executive are united in their support for Adam in this role," it added.